Method and apparatus for measuring strain

ABSTRACT

A method of, and apparatus for, measuring dynamic torque transmitted by a shaft having an axis of rotation is characterised by the steps of: 
     1 locating on the shaft (S) a pair of transducers (T, T1, T2), each comprising an SAW resonator, as a complementary pair so that for a first direction of rotation (K) of the shaft (S) about the axis (A) one transducer (T1) is under compression and the other (T2) in tension and for a reverse direction of rotation of the shaft the one transducer (T1) is in tension and the other (T2) in compression, a signal input (C1) and a signal output (C2, C3) for either each transducer or a single signal output for a signal derived from both transducers, the signal input and signal output or outputs being located at discrete locations on or near the outside of the shaft (S) for rotation therewith, 
     2 causing a driving signal to be applied to the signal input (C1); 
     3 detecting at each or the signal outlet (C2, C3) at least an output resonant frequency of the transducer (T, T1, T2) when driven by the driving signal; and 
     4 processing the output resonant frequency signal of each or both transducers to derive information as to the strain generated in the transducer (T1, T2) induced by stress in the shaft (S) due to dynamic torque transmitted by the shaft (S). 
     The invention further provides for various output signals to be provided relating to dynamic torque with or without temperature compensation and/or representation.

BACKGROUND

This invention relates to a method and apparatus for measuring strainarising from stress. Many applications call for strain measurementincluding static and dynamic loading of structures and components andfor the subsequent derivation of information from such measurement. Thepresent invention is particularly, but not exclusively, concerned withthe measurement of dynamic torque arising when power is transmitted byway of a rotating shaft.

TECHNICAL FIELD

For the measurement of static and dynamic strain conventional resistivestrain gauges are widely used. They are characterised by resistiveelements incorporated into bridge circuit so that changes in resistiveelements due to stress in components to which at least some of theelements are secured are quantified by way of the bridge circuit.

Torque Measurement

A resistive strain gauge can be used for measurement of torquetransmitted by a shaft to which the gauge are securely attached. Sliprings on the shaft are used to feed signal inputs to and to recoversignal outputs from the gauge. Changes in the geometry of the gaugearising from twisting of the shaft during torque transmission aremonitored by way of the slip rings. However the use of slip rings has anumber of disadvantages. Brush drag on the slip rings produces errorswhich are significant for measurement of signals representing lowertorque values and drag can vary with the friction conditions and as wearoccurs. Inertia effects limit the acceleration to which the shaftbearing the slip rings can be subjected. The rings and brushes generateelectrical noise.

The measurement of torques can extend from on the one hand the verysmall torques arising from viscosity associated with the use of smallscale laboratory mixers to, on the other hand, the very large torquesoccurring in transmission shafts of aero and marine propulsion units. Ithas been found that in general slip ring systems are not readily appliedto the measurement of torque of less than about 2 Newton meter.Typically the diameter of the gauge section of a shaft becomes so smallthat insufficient area is available to provide for the mounting ofstrain gauge of suitable size and the size of the electrical noisegenerated results in a signal to noise ratio which prejudices effectiveuse. At the other extreme on very large shafts the use of slip rings andthe associated equipment can lead to access and housing problems.

As an alternative to resistive strain gauges optical torque transducershave become available for the purposes of shaft torque measurement. Inan optical transducer radially extending segmented gratings are mountedon the shaft. Output from an array of light sources passes through thegratings to fall on a bank of photocells. The signal output from thephotocells varies directly with the torque applied to the shaft.Accuracy of measurement is unaffected by the shaft speed or the torquerange. The intensity of the light sources can be monitored to ensurethat photocell output does not vary due to light source variation.

Currently these optical/electronic gauges are widely used for torquesextending in ranges from 0 to 10 milli Newton meters to 0 to 5000 Newtonmeters and for rotational speed between 0.5 to 30000 rpm. While thetechnical advantages of the optical electronic systems are substantialthey are costly. In particular the segmented gratings are expensive tomanufacture. In addition a periodic need to replace lights in the arrayimposes a design constraint inasmuch as the gauge needs to be accessiblefor maintenance and calibration.

BACKGROUND ART

In what follows reference is made to a `surface acoustic waveresonator`. Such a resonator is made up of a microstructure deposited ona piezoelectric substrate. The microstructure is formed by at least onepair of interleaved comb-like (`interdigitated`) electrodes deposited asa thin metal conducting layer on the substrate. FIG. 15 shows a basicmodel of a surface acoustic wave device 10 having input electrode 11interleaved with output electrode 12. The electrodes are a deposit ofaluminum (other good conductors can be used) having a thickness of theorder of 1000 Angstroms. The electrodes 11, 12 are deposited on uppersurface 14 of a piezoelectric substrate 13. Many piezoelectric materialsare suitable for a substrate from flexible plastic polymers to hardmaterials such as ceramic and quartz. Various piezo electric crystalforms can be used such as lithium niobate, lithium tantalate, bismuthgermanlure oxide and gallium oxide. Hereafter such a surface acousticwave device will be referred to as an `SAW resonator`.

In an SAW resonator the application of an electric signal to oneelectrode in the pair causes it to act as a transducer converting theelectrical input signal into an outgoing acoustic wave on the substrate.The other electrode in the pair reverses the process providing anelectrical output signal with the arrival of an acoustic wave on thesubstrate.

An SAW resonator acts as a `high Q` device that is to say one in whichthe selectivity of the circuit is high and narrows the bandwidth passedby the circuit. SAW resonators are among a number of surface acousticwave devices which are widely used in signal processing applicationssuch as delay lines, frequency filters, bandpass filters, oscillators,duplexers and convolvers. SAW devices are the subject of currentresearch and development. A number of publications are availableincluding `Surface Acoustic Wave Devices and Their Signal ProcessingApplications` by Colin Campbell (1989 Edition) published by AcademicPress, Incorporated of San Diego, Calif. USA.

DISCLOSURE OF THE INVENTION

In its broadest concept the present invention is concerned with a methodand apparatus of strain measurement utilising an SAW resonator. In agiven fundamental unstrained state of the substrate and the electrodesmounted upon it a signal input fed to the input electrode results in thetransmission of a surface acoustic wave to the output electrode with acharacteristic resonant frequency output signal from the outputelectrode. In the event the substrate is subject to strain then theconsequent change in the relative geometry of the electrode arrayresults in the resonant output frequency being subject to change whichcan be detected and related to the amplitude of the strain or a functionof it. This is capable of wide application since given a source ofstress which can be transmitted into strain in the substrate the sourcecan analysed by way of signal changes resulting from geometry changes inthe SAW transducer.

The operational frequencies of an SAW resonator can be selected anywherein a wide frequency range extending from a few megahertz up to fewgigahertz. The higher the frequency used the smaller the enveloperequired for the transducer which can be of particular benefit in strainrelated applications where available space is limited. The resonantfrequency used depends on a number of factors including the geometry ofthe electrodes and properties of the substrate material. The velocity ofthe surface wave varies with the temperature of the substrate material.The very small sizes in which an SAW resonator can be made facilitatesits use as a strain measuring device in a wide range of applications.

Coupling between the electrodes can be by `surface acoustic` (also knownas Rayleigh) waves. For such waves the substrate needs to have a smoothpropagation surface for two reasons in particular. Firstly surfacedefects could cause breaks in individual parts of the electrodesaffecting frequency response. Secondly the surface wave energy isconcentrated within a layer one or two wavelengths thick.

Another acoustic propagation mode which can be used to couple theelectrodes are `surface skimming bulk` waves. These extend more deeplyinto the substrate than the surface acoustic waves and consequently thesurface skimming bulk waves have higher losses than arises with thesurface acoustic mode. However the bulk waves are less sensitive todefects in the substrate surface.

The choice of operating mode will depend on the strain measurement to beundertaken.

An SAW resonator can be used in a system where signal inputs to thetransducer input and signal outputs from the transducer are transmittedby non-contact coupling (such as by inductive, capacitative or radiowave means) to an external control system. The provision of anon-contact coupling where the electrodes have no direct electricalconnection provides a number of advantages particularly when there is aneed for intrinsic safety or where physical connection would affect theresonance to be measured. Such non-contact systems are particularlyconvenient for rotating mechanisms. An SAW transducer can be used inplace of a resistive strain gauge. In any event an SAW strain transduceris capable of a degree of accuracy substantially greater than that of aconventional resistive strain gauge.

The electrode array in an SAW resonator can take a number of formsthough all provide for an accurate and specific relationship betweenoperating frequency and electrode geometry.

A single port SAW resonator requires only two connections for operation.Conveniently such a transducer is used with an amplifier having anegative input resistance characteristic so that a state of oscillationcan be maintained by an impedance change in the SAW resonator.

A two-port resonator has lower losses than a corresponding single porttype, can be made to operate in a multi-mode fashion and has advantageswith regard to phase shift making it applicable to high precisionapplications.

An SAW resonator transducer according to the present invention can beused in a wide range of applications and some of these will now bebriefly described.

LOAD MEASUREMENT

The present invention is applicable for measuring the magnitude ofrotary and static loads. If an article which is to be the subject ofloads comprises or incorporates a piezoelectric material then an SAWresonator electrode array can be deposited upon an area of the materialto form an integral transducer. Two or more such transducers can be usedas will be described hereafter in connection with torque measurement.

PRESSURE MEASUREMENT

The present invention particularly lends itself to measurement ofpressure by utilising a non-contact transmission of signal input andoutput from a static interrogating unit, such as a vehicle body, to atransducer located on a moving component, such as an inflated road tireof the vehicle. In this way pressure and other parameters, such as wear,can be periodically or continuously monitored.

TEMPERATURE MEASUREMENT

Evaluating temperature in connection with torque measurement will bediscussed later in relation to processing output signals from two ormore SAW transducers. The invention can be used in connection with abi-metallic strip where the coefficient of expansion of one metal makingup the strip differs from that of the other. In the event of atemperature change the strain on one side of the strip will becompressive and on the other side tensile. Consequently by mounting anSAW resonator on each strip the frequency difference between theiroutputs will be the representative of the ambient temperature due tomaterial strain. Alternatively the sum of the frequencies will be ameasure of temperature due to material expansion. For such a deviceoperating power levels lower than 1 mW can be used which, in combinationwith a non-contact coupling, would meet intrinsic safety requirements.

GYROSCOPIC DEVICE

If a hemisphere is brought to a resonant condition and rotated it isknown that an imposed strain pattern will also rotate relative to thehemisphere at one third of the rate of rotation. During resonanceopposite points on the hemisphere move outwardly while opposite points90 degrees around the rim move inwardly. The process then reverses. Bylocating SAW resonators at 90 degree separation around the hemispherethis strain can be detected by one resonator in compressive strain andthe other in tensile strain to establish the null position. Such agyroscopic device has a number of navigational and guidanceapplications. In particular given that the hemisphere is normallymanufactured in a piezoelectric material (such as quartz or a ceramic)it would be possible to deposit electrode array directly onto thehemisphere material. As a consequence the strain and temperatureinformation derived from such an assembly could be used to improve theperformance of gyroscopic devices.

MAGNETIC FIELD MEASUREMENT

A device according to the invention can be used in a method ofmeasurement of magnetic fields by detecting strain differences in anelastic piezoelectric material (such as quartz) which is subject tostrains induced by reactive Laplace forces produced by current flow in aconductor placed on the material. Such strains are orthogonal to themagnetic field, In practice the strain is converted to a current ofproportional amplitude and phase. This in turn induces strain whichwould in turn produce feedback whose frequency would be proportional tothe magnetic field. Such a device is a vector magnetometer and threesuch devices placed orthogonally would be required for total fieldmeasurement. One advantage of such a device would be the inherentstability for both temperature and time dependent factors ofpiezoelectric materials such as quartz. Other materials, such as galliumarsenide, which are both piezoelectric and a semiconductor provide forthe integration of electronic amplifiers or other circuit elements to beintegrated into a transducer in addition to the SAW resonating elements.

BRIEF DESCRIPTION OF DRAWINGS

A number of exemplary embodiments of the present invention will now bedescribed with reference to the accompanying drawings of which:

FIGS. 1 is a diagrammatic view of a transducer forming a firstembodiment;

FIG. 2 is a circuit diagram of a transducer similar to those describedin connection with FIGS. 1 combined with an amplifier;

FIG. 3 is a diagrammatic view of a twinned pair of transducers of thetype shown in FIG. 1;

FIG. 4 is a circuit diagram of a twinned pair of transducers similar tothose described in connection with FIG. 3 combined with an amplifier;

FIG. 5 is a circuit diagram incorporating the components described inconnection with FIG. 4 coupled to additional components to provide forcompensation and correction;

FIG. 6 is a circuit diagram showing the coupling of transducersdescribed in earlier figures to an output device;

FIG. 7 is an application of the system shown in FIG. 6;

FIG. 8 is a further development of the circuitry described in connectionwith FIG. 7;

FIG. 9 is circuit diagram of an integrated microcircuit unit related tothat shown in FIG. 5;

FIGS. 10 and 11 diagrammatic views of a second embodiment;

FIG. 12, 13 and 14 are diagrammatic views of a third embodiment; and

FIG. 15 is a diagram of an SAW device.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

The following exemplary embodiments described in relation to FIGS. 1 to9 show the use of one or more SAW resonators to measure dynamic torque.The features discussed are also applicable to other applications.

To measure torque requires that the substrate of the, or each, resonatoris securely attached to the shaft or other strain transmitting memberunder test. This can either be directly or by way of an interfacebetween the substrate and the test piece such as might be needed, forexample, where the resonator is encapsulated for protection of theresonator components against a hostile environment. Torque (radialstrain) is measured by a change in the output frequency of the resonatorarising from a change in shape of the substrate element and so in therelative positions of the electrodes deposited upon the substrate. Theradial strain is induced by the stress in the shaft under test and isproportional to the applied torque.

FIG. 1

This shows a shaft S to which has been secured the substrate of an SAWresonator transducer T. The centreline C of the transducer T is locatedat 45 degrees to longitudinal axis A of the shaft S. If the appliedtorque is applied clockwise from the right hand end of the shaft S (asshown by arrow K) the transducer T is placed in tension. If the torquedirection was reversed the transducer T is placed in compression.

FIG. 2

Transducer T of the type shown in FIG. 1 is connected with an amplifierA. The circuit will oscillate at a given frequency when the amplifierprovides for the correct phase shift for, and with enough gain toovercome losses occurring in, the resonator and coupling.

The temperature coefficient of an SAW resonator can be very low. Itsfrequency of oscillation can be up to about 1 Giga Hertz (10⁹ Hz). Atypical frequency would be 500 MHz and therefor a change of 0.1% wouldproduce a frequency change of 500 kHz (5×10⁵ Hz).

FIG. 3

For practical applications two SAW resonator transducers T1 and T2 aremounted as shown on the shaft S with their centrelines C1 and C2 atright angles to one another. Torque applied to the shaft in clockwisedirection shown by arrow K will induce compressive stress in transducerT1 and tensile stress in transducer T2. Any temperature changes wouldapply equally to both elements. An alternative paired arrangement iswith the two transducers mounted on opposite sides of the shaft againwith their centrelines at right angles to one another (and at 45 degreesto the axis of the shaft).

FIG. 4

A pair of transducers T1 and T2 are mounted on a shaft as shown in FIG.3. Transducers T1 and T2 are coupled with, respectively, amplifiers A1and A2 which provide output signal frequencies, respectively, F1 and F2.The output signal are fed to a mixer M having an output F. The outputfrequency F will be F1+F2 and F1-F2. Ignoring the F1+F2 component atypical frequency difference of 1 MHz would be generated for a change of0.1% due to applied torque. Output from the transducers is onlyproportional to torque and with no torque being transmitted F1-F2 willbe close to zero. Changes in the geometry of the transducers T1 and T2due to temperature or endload effects will apply equally to both so thatthe net result will be zero.

The tensile strength of virtually any material is temperature dependent.Consequently a knowledge of the ambient temperature will enablecorrection or compensation to be made. Thus the output F, (F1+F2), ofmixer M will, if compared with a reference, produce a signal frequencyproportional to temperature of the material to which the substrate ofeach transducer is attached.

FIG. 5

This circuit incorporates one similar to that of FIG. 4. Componentsreferred to in connection with FIG. 4 are given the same reference inFIG. 5. In addition FIG. 5 shows a further mixer M2 and a referencesignal generator R to enable temperature corrections to be applied.Output frequency F1+F2 (=A) of mixer M is fed to mixer M2 together witha signal B, typically 1 GHz, from generator R. As a result mixer M2provides as output sum A+B or A-B. The difference signal is at afrequency related to temperature and can be used to correct scalefactors in precision applications.

As SAW devices operate in ultra high frequency regions finite impedancematching is required to obtain the most efficient coupling. In thepresent context this amounts to an impedance of the order of 50 ohms.The device described in connection with FIGS. 3, 4 and 5 aredirectionally insensitive with a typical loss of 18 dB.

FIG. 6

This shows how energy is coupled to each of the SAW resonatortransducers T1 and T2 by way of a coil and a probe. Coils C1-C3 are eachfitted to the shaft S for rotation with it and are coupled totransducers T1 and T2. The coils can be designed to give more than onepeak amplitude per shaft revolution as would be appropriate for lowspeed applications such as ship propeller shafts. Static probes P1-P3are mounted adjacent the shaft. The necessary matching impedance is 50ohms for the coils C1 to C3 and to probes P1 to P3. Amplifiers A1 and A2must provide adequate gain for their associated circuitry to oscillatetaking into account the space loss between each coil and probecombination. Each amplifier is provide with automatic gain control(`AGC`) to accommodate reasonable system losses such as can arise fromchanges in distance between coil and probe. Power dividers R1, R2 serveto regulate frequencies fed to mixer M. The SAW resonators do not haveto be harmonically related. Any frequency difference out of mixer M canbe reduced by making use of a further mixer.

FIG. 7

This shows a practical embodiment of a control arrangement based on FIG.6. Since the transducers are positioned on the shaft as shown in FIG. 6they are consequently not shown in FIG. 7. Coils C1 to C3 serve totransmit signals to probes P1 to P3 as described in connection with FIG.6.

Diodes D1 and D2 change impedance with applied voltage to provide an AGCeffect. On the output voltage from an amplifier increasing it isrectified and used to provide a control current to pin diodes D1, D2 soreducing diode resistance and so causing levelling of the amplifieroutput which in turn produces a constant frequency output from theresonator.

Further information can be provided from the output of the transducerarrangement. Thus shaft rotational speed can be represented by amplitudemodulation of pin diode control current if ciol C1 is made eccentricwith respect to the shaft so generating an output related to each shaftrevolution. For slowly rotating shafts (such as a ships propeller shaft)a coil or coils can used to generate a number of outputs per shaftrevolution (an output for a given angular displacement of the shaft).

FIG. 8

Circuitry shown and described in connection with FIG. 6 and 7 are herecombined together with additional features. Where a component shown inFIG. 8 is similar in form and function to one shown in FIG. 6 and/or 7it is given the same reference.

Shaft S is equipped with transducers T1 and T2 which are coupled by wayof coils C1 and C2 and probes P1 and P2 to the remainder of the controlcircuitry. In this case the output frequency of T1 is 250 MHz and thatof transducer T2 500 MHz. Amplifier A1 feeds its output of 250 MHz tofrequency doubler D from which an output signal of 500 MHz is fed toMixer M. Amplifier A2 feeds its output frequency of 500 MHz to mixer Mdirectly. Output of mixer M, representing (500+500) MHz and (500-500)MHz is split. The difference signal (representing a torque signal) isfed directly to a microprocessor MP by way of line 10. The sum signal(providing a temperature signal) is fed to a second mixer M2 by way ofline 11. The mixer M2 also receives by way of line 12 a reference inputsignal of 1 GHz. The output of the mixer M2 representing a temperaturesignal which is fed by way of line 13 to the microprocessor MP to enablethe microprocessor to provide for a temperature compensated output. As aconsequence the microprocessor MP provides an output to display Wshowing the speed of rotation, the transmitted torque, and the powertransmitted by the shaft S under test with due temperature compensation.

The transducer pairs shown in FIGS. 3, 6 and 8 are mounted at 90 degreesto one another and 45 degrees to the shaft. Other mounting angles can beused. Typically to enable shaft load measurements to be checked the twotransducers while being mounted at right angles to one another could bemounted with one transducer parallel to the axis of the shaft. In thisway the transducer parallel to the shaft axis will be sensitive to theeffect of axial loads on the shaft.

The embodiments make use of probe/coil coupling between the shaftmounted transducers and the control circuitry. However for use in staticmeasurements the transducers could be coupled by wires to the controlcircuitry.

FIG. 9

A method used to ensure that an acoustic wave resonator operatescorrectly must provide for adequate gain and phase stability. Theserequire close control of circuit parameters including coil couplingcharacteristics. Such control is readily met in a purpose builttransducer. However where the resonator has to be added to an existingmechanical assembly further components such as a torque or load sensorhave to be added. A solution is to combine the required electronics withthe acoustic wave resonator unit itself. FIG. 9 shows a circuit diagramfor such a combined unit which incorporates SAW resonators 91, 92,amplifiers 93, 94 and mixer 95 in a similar way to that discussed inrelation to FIG. 4. The output of mixer 95 (at a frequency of about 1MHz) is fed by way of inductor 96A, capacitor 96B to coil 97 which iscoupled by capacitor 98A, diode 98B and impedance 98C to provide a DCsupply to mixer 95 and the amplifiers 93, 94. Electrical energy iscoupled from an external source by way of coil 97 and duly rectified andregulated by way of capacitor 98A, diode 98B and impedance 98C.Frequency f1 from amplifier 93 and frequency f2 from amplifier 94 aremixed in mixer 95 whose output is returned to coil 97. There wouldnormally be a significant frequency difference between the excitationsource and the return signal from mixer 95. The use of a micro circuitenables the relevant electronics to be housed in the same envelope asthe resonators 91, 92. Such a package provides a readily added componentfor attachment to an existing shaft or other mechanism subject to strainwhich is to be measured.

FIG. 10, 11

FIG. 10 is a part sectioned elevation and FIG. 11 a plan view form aboveof a gyroscopic unit. A resonatable hemisphere 101 manufactured from apiezoelectric material (in this case quartz though a material such as aPZT ceramic could be used) and located in a closed and evacuated housing102. Rim 103 of the hemisphere 101 is located in an annulus bounded onits outside by an array 104 of discrete forcers electrodes and on itsinside by a complimentary array 105 of forcer electrodes. The arrays104, 105 are mounted at the same horizontal level as rim 103. Thehemisphere 101 has mounted on it SAW resonator devices 107, 108 whichcan be coupled by, respectively, probes 109, 110 mounted on the walls ofhousing 102 to a source of input energy and frequency sensors. Cableoutlet 111 provides an airtight passage for cables into the housingconnected to forcer electrodes arrays 104, 105 and probes 109, 110.

Hemisphere 101 is brought to a resonant condition by way of forcerelectrodes arrays 104, 105. The hemisphere 101 is then rotated aboutaxis X and it is known that an imposed strain pattern will rotaterelative to the hemisphere 101 at one third of the rate of rotation ofthe hemisphere. During resonance opposite points on the rim 103 of thehemisphere 101 will move outwardly while opposite points 90 degreesaround the rim 103 move inwardly so generating strain in the hemisphere.The process then reverses. By locating SAW resonators 107, 108 with anangular separation of 90 degrees around the hemisphere 101 this straincan be detected by one SAW resonator 107 in compressive strain and theresonator 108 in tensile strain to establish the null strain position.Such a gyroscopic device has a number of navigational and guidanceapplications. In particular by manufacturing the hemisphere 101 in apiezoelectric material the SAW resonators 107, 108 directly onto thebody of the hemisphere 101. The use of SAW resonators in thisapplication provides for high sensitivity and good frequency response.They enable a much smaller resonator to be manufactured than one using alower resonance frequency and capacitive detection of the null position.As a consequence the strain and temperature information derived fromsuch an assembly could be used to improve the performance of gyroscopicdevices. A third SAW resonator can be located on the hemisphere 101 toprovide a signal indicative of the direction of motion of strainpattern.

FIG. 12, 13, 14

These show the use of an SAW resonator to measure pressure change bymaking attaching SAW resonators to a diaphragm which is subject to thepressure change. Typically the diaphragm has one resonator on one sidein tension and another resonator on the other side in compression.. Thesignals are then mixed to produce temperature compensation for thestrain output in a manner akin to that described in relation to FIG. 8where the sum output from mixer MP provided a temperature signal.Various methods of coupling can be used.

FIGS. 12, 13 show a car wheel 121 on which is mounted a pneumatic tire122 (shown in section in FIG. 12) whose interior 123 is inflated to apredetermined pressure. The wheel 121 is mounted in a known manner on asuspension unit 124 attached to car body 125. On wheel rim 126 there ismounted an SAW resonator transducer assembly 127 which is described inmore detail in relation to FIG. 14. Output signals from the assembly 127are periodically sensed by way of probe 128 coupled to a processor 128Amounted on structure of the car body in the vicinity of the wheel 121.In FIG. 13 the assembly 127 is shown angularly offset from probe 128.

FIG. 14 shows the juxtaposed transducer assembly 127 and the probe 128in more detail. Wheel rim 126 is pierced by aperture 130 through whichextends body 131 of the assembly 127 into the interior 123 of the tire.The body 131 is secured on the rim 126 by way of nut 132 and O-ring 133serves to maintain a fluid tight seal between flange 134 integral withbody 131 and the rim 126. The body 131 has an end aperture 135 openinginto cavity 136. The cavity 136 is isolated from the remainder of bore137 in the body 131 by a flexible diaphragm 138. The diaphragm 138 hasSAW resonator transducer 139 deposited on its upper side and a secondtransducer 140 deposited on its lower side. Circuit capsule 141 ispositioned in bore 137 so as to be adapted to receive and transmit databy way of coil 142 to a corresponding coil 143 mounted in probe 128. Thedata comprises the output frequencies of resonators 139, 140 adapted toprovide diaphragm information as to strain, directly related to tirepressure, and the temperature of air within the tire interior 123 asdescribed in connection with FIG. 8. Various methods are available forcoupling the tire operating parameters to a probe. The arrangementdescribed in connection with FIGS. 12, 13, 14 will only produce anoutput when transducer 127 is in close proximity to probe 128 that is tosay once for each revolution of the wheel. With a coil corresponding tothat of coil 142 positioned on inner rim 144 of the wheel 121 and a coilcorresponding to probe coil 143 mounted around axle 145 then acontinuous information output is obtained. Correct operation of theprobe is detected by monitoring the signal level. A single readout unitin the vehicle receives the output from pressure transducers of the typedescribed mounted on each of all the wheels of the car. The readout unitprovides for the displays of any selected tire pressure and temperature.If necessary the selected reading can be displayed in relation topre-set operating limits. The exemplary embodiments, and particularlythat described in connection with FIG. 8, provide for strain measuringsystems offering substantial advantages over existing devices. Apartfrom information on tire pressure and temperature the tire canincorporate a transducer serving to transmit a signal relating to tirewear by way of the probe system.

The transducer of the present invention in addition to being small insize and capable of very accurate outputs provide for low manufacturingcost and to large scale production. A strain gauge transducer based onthe SAW device makes use either of existing components or, as in thecase of the SAW itself, of an electrode deposition process which iscapable of being manufactured more cheaply and with greater inherentaccuracy than existing transducer components (such as resistance gaugesor apertured disks). In addition the mounting of the components is morereadily achieved than with existing strain gauge or optical torquemeasuring devices.

In one form the transducer would be encapsulated to protect theelectrodes and working circuitry. The transducer is attached to the itemwhose stressing is of concern typically by welding or adhesive to ensurethat the substrate material is as tightly secured as possible so thatthe maximum transmission of strain occurs from the test piece to thesubstrate.

An SAW resonator can be made small in size and mass. As a consequenceinertia effects are minimal in contrast to currently available systemswhich in the main involve the mounting of inertially significantcomponents. Because of the low size and mass the system can accommodatea wide variety of operating conditions. The proposed SAW resonatorsystem provides for high accuracy and sensitivity achieved by the use ofwhat amounts to frequency modulating techniques making use of solidstate components having high reliability and of low power consumption.The associated control and processing equipment utilise available signalprocessing methods and components which are readily interfaced withexisting digital processes and equipment. Typically in the case oftorque measurement there are virtually no limitations imposed by theshaft mounted part of the system on the speed at which the shaft undertest can be rotated or at which it can be accelerated or decelerated.The system will also provide valid data from start-up from zero shaftspeed on start up and for very low shaft speeds.

The proposed SAW resonator system of the present invention is inherentlysafe since only signal strength power is used. This contrasts with, forexample, currently available torque measurement by way of an opticalsystem requiring the use of sufficient power for a plurality of lamps.

We claim:
 1. A method of measuring dynamic torque in a rotatable shaftwherein at least a pair of surface acoustic wave generators are disposedon a substrate, and are disposed to distort in response to the torque tobe measured such that the distortion serves to alter the frequencycontrol of the surface acoustic wave generator with a consequent changein output frequency comprising the steps of:a) locating on a shaft (S) apair of transducers (T1, T2), each comprising an SAW resonator, mountedas a complementary pair so that for a first direction of rotation (K) ofthe shaft (S) about an axis (A) a first transducer (T1) is undercompression and the second transducer of said pair (T2) is in tension, asignal input (C1) and a signal output (C2,C3) for each transducer, thesignal input and signal outputs being located at discrete locations,said discrete locations being one of on and near an outside of the shaftfor rotation therewith; b) causing a driving signal to be applied to thesignal input (C1), which is carried out by way of a signal transmitter(P1) coupled to the signal input by one of an inductive, capacitive andradio wave means of low power; c) detecting at each signal output(C2,C3) at least an output resonant frequency of the correspondingtransducer (T1, T2) when driven by the driving signal by way of a signalreceiver (P2) coupled to the signal outputs (C2,C3) by way of one of aninductive, capacitive and radio wave means of low power; and d)processing the output resonant frequency signal from each transducer(T1, T2) to derive information as to the strain generated in thetransducers (T1, T2) induced by stress in the shaft (S) due to dynamictorque transmitted by the shaft (S).
 2. A method of measuring dynamictorque according to claim 1 wherein the step of detecting an outputresonant frequency from the first transducer (T1) provides a firstsignal frequency and the output resonant frequency (F1) from the secondtransducer (T2) provides a second signal frequency (F) at least one ofthe first and the second signals being processed (R1, R2) prior to amixing process (M).
 3. Apparatus for measuring dynamic torquetransmitted by a shaft having an axis of rotation wherein a pair oftransducers, each comprising an SAW resonator, are located relative to ashaft as a complementary pair comprising:a pair of transducers (T1, T2)being located on the shaft; a signal input (C1) comprising a signaltransmitter (Pl) coupled to the signal input (C1) by one of aninductive, capacitive and radio wave means of low power; a signal output(C2) comprising a signal receiver (P2) coupled to the signal output (C2)by way of one of an inductive, capacitive and radio wave means of lowpower, each transducer being located at discrete locations, saiddiscrete locations being one of on and near the outside of the shaft (S)for rotation therewith, each transducer (T1, T2) comprising apiezoelectric substrate (13) having mounted on one side a pair ofinterdigitated electrodes (11, 12), a first electrode (11) of the pairbeing connected to the signal input (C1); a generator (A1, A2) forapplying an input signal at a predetermined frequency (F1, F2) to thesignal input (C1); a signal processing mixer (M) for receiving a signalfrom the signal output (C2); and a processor (R) whereby changes in theoutput signal arising from strain applied to the substrate (13) can bederived.
 4. Apparatus for measuring dynamic torque according to claim 3wherein the signal processing mixer (M) is adapted to receive the outputresonant frequency (F1) from a first transducer (T1) and the outputresonant frequency (F2) from the second transducer of said pair (T2) andto mix both signals to produce a composite signal derived from both thesignals (F1, F2).
 5. Apparatus for measuring dynamic torque according toclaim 3 wherein the signal processing mixer (M) provides signal outputsof a sum and a difference signal derived from the outputs of thetransducer pair (T1, T2), the difference signal being a function ofmeasured torque, and the sum signal being a function of the ambienttemperature in the region of the transducers.
 6. Apparatus for measuringdynamic torque according to claim 5 wherein the difference signal andthe sum signal are fed to a common processor (MP).
 7. Apparatus formeasuring dynamic torque according to claim 3 wherein the transducers(91, 92), generators (93, 94), and a mixer (95) are packaged as a unitfor mounting on a shaft whose torque is to be measured, the signaloutputs from each transducer being combined by way of the mixer andthereafter fed to a common output unit (97) for transmission from theunit (95).